Control and indication system for explosion suppressors



C. F. ROCKWELL ATTORNEYS Feb 18, 1969 CONTROL AND INDICATION SYSTEM FOR EXPLOSION SUPPRESSORS u w m M N In. 3 1 F m m a F F. m A H I C W 9 1 m m w I m .i '55, 1 x 3 w a |H L 1 V w 0 3 a ml G 2 .v w; Q 2 Ii! I II H United States Patent 6 Claims ABSTRACT OF THE DISCLOSURE The specification describes an explosion suppression ssytem particularly useful in airplanes. A radiation sensitive device such as a photovoltaic cell is mounted in the vent conduit leading between a fuel surge tank in the aircraft wing and the fuel system vent. It is shielded from external radiation by the vent conduit shape. The cell is connected to supply an input signal to an amplifier Whose output signal is applied across a tunnel diode. The tunnel diode in turn is connected between the gate and cathode terminals of a silicon controlled rectifier. A source of voltage is connected directly to the anode of the silicon controlled rectifier and the cathode is connected directly in series with the actuating circuit of the explosion suppressors. When a sufficiently large signal is applied to the tunnel diode to switch it across its valley region, it fires the silicon-controlled rectifier, thereby supplying current to the explosion suppressors. When the suppressor fires, its actuating circuit opens thereby opening the circuit of the silicon controlled rectifier and thus causes it to cease conducting; thereby arcing across the open ends of the wires of the explosion suppressor actuating circuit is prevented.

My invention relates to explosion suppression, and more particularly to a novel control and indicating system for explosion suppressors.

Explosion suppression systems have been developed for preventing explosions from taking place in spaces in which a combustible mixture of gases exists or may be formed. These systems comprise frangible containers of inert gas or liquid, opened by an electric-ally actuated squib. The effectiveness of such a system depends upon detecting an incipient explosion and firing the squib to expel the inerting fluid into the burning mixture before the pressure in the protected space can rise to an explosive level. It has been found that the rate of rise of pressure in a confined space following the ignition of such potentially explosive mixtures as hydrocarbons and air is sufliciently low that the detection of an incipient explosion can readily be accomplished by the use of a photocell responding to radiation from a flame front indicating an incipient explosion, and a switch controlled by the cell to supply actuating current to the electrical squib-firing circuit path of the suppressor. Such systems have proven quite satisfactory for many purposes, but a number of problems have been encountered in their use, particularly when the protected space is vented to the atmosphere, as is the vapor space in a vented fuel tank.

One of these problems arises from the interruption of the actuating path through the explosion suppressor by the flow of current igniting the squib. Thus, when the suppressor is actuated, a voltage will appear across the terminals of the suppressor. Since the nature of the break in the actuating circuit path is somewhat difficult to control, there is some chance that a spark may jump the wires which carried current to the squib and re-ignite the mixture of gases after the protective inerting gases have been dissipated. It is an object of my invention to eliminate this hazard.

3,428,130 Patented Feb. 18, 1969 Explosion suppressors are important today in a wide variety of environments, but were originally developed to protect aircraft fuel tanks from explosion when pierced by projectiles in combat. As has recently been discovered, another danger of explosion exists in aircraft even though the fuel tanks remain intact and the aircraft is operated as intended. Many commercial and military aircraft contain a plurality of sets of tanks, and have each set of tanks vented by way of a surge tank connected to the individual tank vents and provided with a vent conduit leading to a vent port faired into the skin of the aircraft in an aerodynamically neutral manner.

The surge tank returns liquid fuel that may splash into the tank vents, or condense therein, to the main tanks, and serves to vent the fuel storage system to the atmosphere. At some point outside of the aircraft, as the aircraft gains altitude, or inside the vent conduit, as the aircraft descends, there will exist a combustible mixture of gases. It may happen that this mixture is ignited, as by lightning, causing a flame front to propagate into the tank and produce an explosion. The second object of my invention is to greatly reduce the risk of loss of aircraft from this cause.

In any explosion suppression system, and particularly in an aircraft explosion suppression system, it is essential that the means for detecting an incipient explosion be as sensitive and rapid in response as possible. It is equally essential that the detection means respond only when an explosion impends, as once the suppressor has been actuated and after the inert gases have been dissipated, there will be no protection against explosions until a new suppressor can be installed. It is the third object of my invention to increase the sensitivity of explosion suppre-ssor systems While reducing the risk of false operation.

It is particularly important in aircraft explosion suppression systems that a reliable indication be provided when a suppressor has been actuated. A false indication would require an unnecessary disassembly procedure that would be wasteful and expensive. On the other hand, failure to indicate the operation of a suppressor might allow the aircraft to be operated without protection until its next major overhaul. The fourth object of my invention is to improve the reliability of indicators for explosion suppression systems.

Briefly, the aircraft explosion suppression system of my invention comprises one or more electrically operable explosion suppressors amounted in the surge tank of an aircraft fuel storage system, a radiation responsive incipient explosion detector mounted in the vent conduit, and a visual indicator mounted in the outer surface of the aircraft where it can readily be seen by maintenance personnel. If not originally so designed, the vent conduit should be modified to bend sharply away from the vent opening so that only the portion directly adjacent to the vent can be exposed to direct external radiation. To further reduce external radiation, the inner surfaces of the conduit should be covered with a light-absorbent coating of black paint or the like, preferably with a matte finish. The detector may thereby be made quite sensitive without the risk of false operation by external radiation.

The indicator preferably comprises an electrical actuating element interrupted by the passage of current operating the indicator. If indication is not required, the circuit may be fused in a conventional manner. The actuating elements of the indicator and suppressor or suppressors are connected in series with a source of voltage and an electronic switch controlled by the detector. Means are provided for closing the switch, when an incipient explosion is detected, until the suppressor or suppressors have operated, and opening the switch as soon as the suppressor circuit is broken so that the source of voltage does not appear across the terminals of an operated suppressor.

The construction and mode of operation of the control and indicating system of my invention will best be understood from the following detailed description, together with the accompanying drawings, of a preferred embodiment thereof.

In the drawings, FIGURE 1 comprises a sketch in plan, with parts broken away, of a portion of an aircraft wing incorporating the control and indicating system of my invention;

FIGURE 2 is a schematic cross-sectional elevation of an indicator forming a part of the system of FIGURE 1, taken essentially along the lines 2-2 in FIGURE 1; and

FIGURE 3 is a schematic wiring diagram of the system of FIGURE 1.

In FIGURE 1, I have shown the tip portion of an aircraft wing, having a lower skin surface 3 broken away to show a surge tank 4 mounted between a front spar 5 and a rear spar 6. The surge tank 4 is connected to fuel tanks, not shown, through a series of interconnecting conducits 7. The tank 4 is connected to a vent port 8 defined by a flange 9 faired into the skin 3 and secured to the wing structure by conventional means, not shown. The vent conduit 10 is bent into somewhat of an S shape, as is conventional. Advantage is taken of the fact that the fitting 11 forming a portion of the conduit connecting with the port 8 bends sharply away from the normal to the plane of the vent, whereby direct external radiation only bathes the portion of the conduit immediately adjacent the vent port. For the purposes of my invention, external radiation is preferably still further reduced by coating the insides of the portions 10 and 11 of the vent with a lightabsorbent coating such as a matte finish black paint.

A radiation detector 12 is enclosed in a housing mounted in any conventional manner in the wall of the vent conduit 10 to be exposed to radiation, in the visible and infra-red range, caused by a flame front indicating an incipient explosion. One or more electrically operated explosion suppressors, such as the suppressors 13 and 14, are installed in the wall of the surge tank 4. The number of suppressors would be determined in practice by the size of the surge tank in the manner known to those skilled in the art. Preferably, dibromotetrafluoroethane is the inerting liquid used in the suppressor, and the suppressor contains an electrically ignited charge of explosives, such as a conventional blasting cap, to burst the container and to expel the suppressant fluid, stopping the flame front and effectively preventing explosion. Location of the detector 12 in the conduit is preferred, both because the conduit, and the region just outside of the vent port, are the most probable sites of ignition, and because additional time for igniting the suppressors is gained while the flame front is propagated down the vent conduit.

A visual indicator 15 is preferably located in the lower surface of the wing, about a fourth of the chord forward from the trailing edge, and reasonably close to the suppressors and detector to keep the wiring short. Any conventional electrically actuated visual indicator may be employed Within the scope of my invention in its broader aspects. However, I prefer an indicator of the type described and claimed in US. Patent No. 3,303,307 issued Feb. 7, 1967, to Milton J. Morrisette for Electrically Actuated Visual Indicator for Fuel Tank Protection Systems and the Like, and assigned to the assignee of this application. To facilitate an understanding of the apparatus of my invention in its preferred embodiment, such an indicator has been shown in FIGURE 2 and will next be described.

Referring to FIGURE 2, the indicator 15 comprises a conventional electrically operated indicating capsule 16 removably mounted in a housing 17 of any suitable conventional non-magnetic metal, plastic or the like. The housing 17 is formed with a reduced axial passage 18 to receive and guide the base of the capsule 16. Behind the passage 18 is an opening generally designated 19,

shaped to receive a suitable standard connector. The front of the housing 17 forms a parabolic reflector having an inner surface 20 silvered or polished to be highly reflective. The housing 17 is formed integrally with a mounting flange 21 provided with apertures, as at 22, to receive screws for mounting the indicator on the wing structure. A transparent cover plate 23, of glass, plastic, or the like, covers the reflector and is held in position by any suitable means, here shown as a retaining ring 24 held by a resilient O-ring 25.

The indicating capsule 16 comprises a transparent capsule 26 of glass, plastic or the like, extending into the reflector in a region including the focal point of the reflector. Within the capsule is a quantity of colored powder 27, preferably a bright fluorescent yellow-orange pigment, covered at the end adjacent the reflector by white paper or paint. Also enclosed in the capsule is a charge 28 of explosive, such as lead styphnate or the like, arranged to be detonated by current supplied to terminals 29. The terminals 29 are connected by a bridge wire extending into the charge 28 and broken by a current sufficient to detonate the charge.

The minimum operating current for the indicator should be about the same as the minimum operating current for the suppressors 13 and 14. In practice, I have employed a number 6 electric blasting cap containing an actuating bridge wire with a nominal resistance of one ohm as the suppressor actuating device. Such a cap requires a minimum firing current of about 0.35 ampere for reliable operation, and indicators such as 16 having similar electrical characteristics are readily available.

In operation, when current is supplied to the terminals 29 to detonate the charge 28, the fluorescent colored powder 27 is blown out to coat the walls of the capsule 26. Since the capsule 26 is located in a region including the focal point of the reflector, the observer sees the cover plate 23 as a clear white or mirror-like disk before actuation of the indicator, and as a fluorescent yellow-orange disk, after the indicator is actuated, greatly enhancing the effect of the capsule indicator 16. In use on one of the larger military or commercial aircraft, the indicator may be many feet above the head of an observer standing on the ground beneath the wing, and the larger indicating area produced by the construction of FIGURE 2 is needed to ensure discovery of an actuated suppressor.

It is to be understood, as previously noted, that if an indication is not necessary or required, the function of the indicator in my circuit may be performed by a conventional fuse.

Referring to FIGURE 3, the detector 12 comprises a photocell 30 such as a silicon solar cell or a lead sulfide cell, or the like, suitably sensitive to radiation in the infra-red and visible regions produced by a hydrocarbon fuel burning in air. When illuminated, the cell 30 sup plies current to a conventional D.C. amplifier 31. As shown, a tunnel diode 32 is connected in the output circuit of the amplifier 31 in series with the activating terminals 29 of the indicator 15 and the fusible actuating wires 33 and 34 of the suppressors 13 and 14.

The tunnel diode 32 is connected between the gate terminal 35 and cathode 36 of a controlled rectifier 37. A silicon controlled rectifier is preferred, but a silicon controlled switch, transistor, or other conventional electronic switch may be employed if so desired without departing from the scope of my invention in its broader aspects.

The anode 38 of the controlled rectifier 37 is connected to the positive terminal of a source of voltage, here shown as a battery 39 having its negative terminal at ground potential. In practice, one or more storage capacitors 40 may be connected across the battery 39, to increase the available current when the controlled rectifier 37 is gated on, and to prevent false operation of the controlled rectifier by voltage transients.

As will be appreciated, the amplifier 31 is desirably of relatively high gain and will generally produce a small,

more or less random, output voltage due to noise and background radiation. A small current may flow through the tunnel diode 32 and the circuit through the indicator 15 and the suppressors 13 and 14 from these causes. The amplifier 31 should be designed, and the tunnel diode 32 selected, such that this current is well below the valley region of the tunnel diode characteristic.

When the photocell 30 is illuminated by radiation signaling an incipient explosion, however, causing the cell to produce an output current, the tunnel diode 32 will be switched across the valley region and the voltage across it will rise abruptly. Gate current will then immediately flow through the gate-to-cathode junction of the controlled rectifier, triggering the rectifier into conduction to supply a large pulse of current through the suppressors 13 and 14 and the indicator 15.

The suppressors and indicator will be operated by this current pulse, and within a few milliseconds of the illumination of the photocell 30, the vapor space in the surge tank and vent conduit will be rendered inert by the expanding suppressant released by the bursting of the suppressor casings. I have found that there is an adequate delay between the application of firing current to the series combination of the indicator and suppressors and the interruption of the circuit to ensure that all three devices will operate before the circuit is broken. As shown in FIG. 3 the indicator is connected in series with the suppressors and is desirably connected, as shown, between the controlled rectifier 37 and the suppressors themselves. This arrangement minimizes the possibility of power being applied to the suppressors once they have been fired.

'It will be apparent that at any time the indicator 15 is operated, the explosion suppressors have either operated or should have operated, and that maintenance is in order. Also, the circuit will be broken in three places by the operation of the indicator and suppressors, making it highly unlikely that sparking potential will exist in the surge tank at the opening of the suppressor terminals. Further insurance against sparking in the surge tank is provided by the controlled rectifier 37. As soon as the circuit is broken by operation of the indicator and suppressors, the controlled rectifier 37 will be cut off, and cannot be gated into conduction again because the circuit for the tunnel diode 32 will also be broken. Accordingly, the supply voltage will not appear beyond the anode 38 of the controlled rectifier 37. As the indicator 15 is plainly visible to the ground crew, when the aircraft lands, the new suppressors can be installed as soon as possible after their actuation to prevent an explosion.

While I have described my invention with reference to the details of a preferred embodiment thereof, many changes and variations will become apparent to those skilled in the art upon reading my description, and such can obviously be made without departing from the scope of my invention.

Having thus described my invention, what I claim is:

11. In an explosion suppression system, an electrically operaJble explosion suppressor adapted to be mounted in a space to be protected against explosion and including an electrically conducting circuit opened by the flow of sufiicient current to operate the suppressor, an electronic switch closed by an applied gate signal when voltage is applied across the switch and opened when no voltage is so applied, a source of voltage connected directly in series with the actuating circuit of said suppressor and said switch to supply operating current to said explosion suppressor when a gate signal is applied to said switch, radiation detecting means adapted to be mounted in a space to 'be monitored for producing an output signal when exposed to radiation characteristic of an incipient explosion, and means controlled by said detecting means for applying a gate signal to said gate terminal when an output signal is produced by said detecting means, whereby a flame irradiating said detecting means causes closing of said switch to actuate said suppressor and said switch is opened by the opening of said actuating circuit to thereby remove voltage from the broken actuating circuit.

2. The apparatus of claim 1, in which said switch is a controlled rectifier having an anode and a cathode terminal connected in series with said actuating circuit and a gate terminal, and in which said means for applying a gate signal to the switch comprises a tunnel diode connected between said gate terminal and said cathode terminal and means responsive to said output signal for supplying sufficient current to said tunnel diode to switch it across its valley region to trigger said controlled rectifier into conduction.

3. In an explosion suppression system, an electrically operable explosion suppressor adapted to be mounted in-a space to be protected against explosion and including an electrically conducting path opened by the flow of suflicient current to operate the suppressor, a controlled rectifier having an anode, a cathode and a gate terminal, a source of voltage connected directly in series with the actuating path of said suppressor and the anode and cathode of said controlled rectifier to supply operating current to said explosion suppressor when gate current is supplied to said gate terminal, radiation detecting means adapted to be mounted in a space to be monitored for producing an output signal when exposed to radiation characteristic of an incipient explosion, and means controlled by said detecting means for supplying gate current to said gate terminal when an output signal is produced by said detecting means when exposed to said radiation.

4. In an aircraft wing having a fuel surge tank and a vent conduit connecting the upper portion of the tank with a vent port in the wing, said conduit extending abruptly away from said vent port in a sense normal to the vent opening to shield the interior of the major portion of the conduit from direct external radiation, a radiation a'bsorbing coating on the inner surface, of the conduit, radiation detecting means mounted in the conduit in a position shielded from direct external radiation for producing a signal in response to radiation indicative of an incipient explosion from flame in the conduit, at least one electrically operable explosion suppressor mounted in the tank and having an actuating circuit extending between a pair of input terminals to said suppressor, said actuating circuit being opened by current sufficient to operate the suppressor, an electronic switch closed by an applied gate signal when voltage is applied across the switch and opened when no voltage is so applied, means controlled by said radiation detecting means for applying a gate signal to said switch in response to said signal, and a source of voltage connected directly in series with said actuating circuit and said switch to supply actuating current to said circuit when a gate signal is supplied to said switch.

5. The apparatus of claim 4, in which said electronic switch comprises a controlled rectifier having an anode, a cathode and a gate terminal, said means for applying a gate signal to said switch comprises a means for supplying gate current to said gate terminal in response to said signal to trigger said controlled rectifier into a conducting state, and said source of voltage is connected directly in series with said actuating circuit and the anode and cathode of said controlled rectifier for supplying actuating current to said circuit when said controlled rectifier is in a conducting state.

6. The apparatus of claim 5, in which said means for supplying gate current to said gate terminal comprises a tunnel diode connected between the gate terminal and the cathode of said controlled rectifier and poled to shunt background current, and in which the gate current supplied in response to said signal is above the level necessary to switch said tunnel diode across the valley region in its characteristic.

(References on following page) 7 8 References Cited OTHER REFERENCES UNITED STATES PATENTS Frenzel and Gutzwiller, Solid-State Thyratron Switch- .2 570 2 0 10 1951 Roflman 1 9 2 S Kilowatts, reprint from Electronics, dated March 2,799,781 7/1957 Joyce et a1. -7 250-214 5 1958- 1/1959 MQQISW 169-72 X M. HENSON WOOD, 1a., Primary Examiner. 3,235,860 2/1966 Vass1l 317-130 2 3 1 7 9 White 7 5 HOWARD NATTER, Asszstant Exammer.

FOREIGN PATENTS US. Cl. X.R.

348,612 1931 Great Britain. 10 16923, 26; 250-214 

